Method and apparatus for time and frequency synchronization of OFDM communication systems

ABSTRACT

A method and apparatus for joint time and frequency synchronization for orthogonal frequency division multiplexing (OFDM) systems. A multitone pilot signal is sent in a designated OFDM symbol period. The receiver synchronizes to the pilot signal in a two-stage procedure. The first stage estimates the frequency offset coarsely with a frequency-domain correlation method and estimates the time offset with smoothed time-domain correlation. In a multipath channel, the smoothed time offset estimate is used to locate a cyclic prefix interval which captures the maximum total signal energy. The second stage improves the frequency estimate with a computationally efficient numerical optimization method.

FIELD OF THE INVENTION

[0001] This invention relates to communication systems utilizingorthogonal frequency division multiplexing (OFDM).

BACKGROUND OF THE INVENTION

[0002] Orthogonal frequency division multiplexing (OFDM) is awidely-used technique for wireless and other types of communications. InOFDM, data is transmitted in parallel over multiple equally spacedcarrier frequencies using Fourier transform methods for modulation anddemodulation. By inserting a guard period or guard interval, referred toas a cyclic prefix, between symbols, data on OFDM subcarriers can bereceived orthogonally with no inter-carrier interference (ICI) and nointersymbol interference (ISI). Eliminating the ICI and ISI mitigatesthe effects of delay spread, making OFDM well-suited to wirelessmultipath channels. Moreover, for wireless channels, OFDM can be usedwith coding to easily exploit frequency diversity and combat Rayleighfading to improve reliable information transfer.

[0003] It is well-known that OFDM systems demand strict timing andfrequency synchronization between the transmitter and receiver. To avoidintersymbol interference (ISI), the receiver must adjust its symboltiming so that the symbol transitions occur within the cyclic prefixesbetween the symbols. In a multipath channel, the cyclic prefix mustcontain the symbol transitions under all signal paths. Also, being amulticarrier system, the OFDM receiver and transmitter need to betightly frequency synchronized in order to avoid intercarrierinterference (ICI).

[0004] Several methods have been proposed for OFDM time and frequencysynchronization. Blind algorithms known in the art generally do not useany pilot training signals and typically exploit the correlation of theOFDM cyclic prefix for synchronization. While blind methods aregenerally not wasteful of bandwidth on synchronization pilots, thesynchronization accuracy is typically not as good as that attained usingpilot-assisted methods. Other known systems utilize pilot-assistedsynchronization methods based on a number of different pilotsynchronization signals.

SUMMARY OF THE INVENTION

[0005] In the present invention, the receiver performs the time andfrequency synchronization using a multitone pilot synchronization signaltransmitted in a designated OFDM symbol period. The multitone pilotsignal consists of discrete tones whose tone frequencies and tonecoefficients are a priority known to the receiver.

[0006] The synchronization from the multitone signal is preferablyperformed in two stages. The first stage uses a coarse frequencydiscretization using F candidate frequency offset estimates. For eachcandidate frequency offset, a smoothed time-domain correlation (TDC)estimation procedure is used to estimate the pilot signal's time offsetand received signal energy. The procedure yields F candidatetime-frequency offset estimate pairs, and the time-frequency estimatecorresponding to the largest detected energy is selected.

[0007] After the first stage is completed, the frequency offset estimateis refined in a second stage by a numerical optimization procedure. Thetime estimate from the first stage and the optimization procedure of thesecond stage finds the frequency offset at which the correlation betweenthe received signal and the pilot signal is maximized. A computationallyefficient method for performing the optimization, described herein, maybe utilized.

[0008] An alternate, simpler implementation of the first stage may alsobe utilized. In such an implementation, the frequency candidates areassumed to be integer multiples of a certain basic frequency. Under thisassumption, an estimate of the frequency with the maximum energy can beselected using a frequency domain correlation method. After thefrequency has been estimated, the time offset is estimated by a smoothedTDC estimation as before.

[0009] In both implementations of the first stage, the TDC correlationcan be implemented with standard Fast Fourier Transform (FFT) methodsfor computational efficiency. The first implementation requires one FFTof the received data plus one FFT for each of the F TDC estimators for atotal of F+1 FFTs. The simplified implementation requires only two FFTs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the drawing figures, which are not to scale, and which aremerely illustrative, and wherein like reference characters denotesimilar elements throughout the several views:

[0011]FIG. 1 is a block diagram of a system of a type utilizing thepresent invention;

[0012]FIG. 2 is an illustrative representation of a general OFDM signal;

[0013]FIG. 3 is an illustrative representation of an OFDM signal beingreceived with both time and frequency offsets;

[0014]FIG. 4 is an illustrative representation of an OFDM signal withmultitone synchronization signals;

[0015]FIG. 5 is a block diagrammatic representation of a two-stagesynchronization system and receiver in accordance with a preferredembodiment of the present invention;

[0016]FIG. 6 is a block diagrammatic representation of a systemimplementing a preferred initial time and frequency offset estimator;

[0017]FIG. 7 is an illustrative representation of multipath signalreception and the relative energies of received signals at variousarrival times;

[0018]FIG. 8 is a block diagrammatic representation of a preferredsmoothed time domain correlation estimator;

[0019]FIG. 9 is a block diagrammatic representation of an alternateembodiment of an initial time and frequency offset estimator;

[0020]FIG. 10 is a block diagrammatic representation of a preferredfrequency offset estimate refinement block; and

[0021]FIG. 11 is an exemplary representation of a multitonesynchronization signal from an OFDM channel estimation pilot signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Other objects and features of the present invention will becomeapparent from the following detailed description, considered inconjunction with the accompanying drawing figures.

[0023] Problem Definition

[0024] With initial reference FIG. 1, there is illustrated an OFDMsystem 10 of a type utilizing the present invention. A transmitter 10and receiver 20 are linked over a channel 12 that imparts an unknowntime and frequency offset on a transmitted OFDM signal. Synchronizationis the process where the receiver estimates these time and frequencyoffsets.

[0025]FIG. 2 illustrates a general OFDM signal 200 arriving with anoffset 30 from the receiver timing. An OFDM signal is a sequence ofsymbols 40 of duration T_(s). Each symbol period contains a data period42 of duration T, and a cyclic prefix period 44 of duration T_(cp). Datais transmitted during the data period 42, while the cyclic prefix 44acts a guard interval between symbols.

[0026] The marks 50 on the time axis 80 of FIG. 2 represent thebeginnings of the OFDM symbol periods as measured at the receiver 20. Asshown in FIG. 2, the OFDM signal 40 arrives with a time offset 30, τ,relative to the receiver symbol timing. Thus it can be seen that theOFDM symbol 40 and receiver symbol period 60 do not begin at the sametime. For proper reception, the time offset 30 must be less than thecyclic prefix length 44, i.e. 0≦τ<T_(cp).

[0027] Referring now to FIG. 3, there is illustrated an OFDM signal 300being received with both time offsets 30 and frequency offsets 70. InOFDM, the data period of each symbol is a linear combination of N tones72 spaced in frequency with uniform spacing 74, shown as 1/T. The OFDMsignal 300 is thus represented in FIG. 3 as a time-frequency grid, witheach column 77 representing the time interval for one OFDM symbol 79,and each horizontal line 78 representing the frequency location of oneof the tones. Data is transmitted in the OFDM signal 300 by modulatingthe tones 72 in the OFDM symbols. An OFDM signal with N tones cantransmit N complex values per OFDM symbol.

[0028] The time axis 80 of FIG. 3 is marked at the points 50 where thereceiver 20 begins the samples for each OFDM symbol 79. The frequencyaxis 82 is marked at the frequencies 87 where the receiver 20 samplesthe tones 72. As shown in FIG. 3, the OFDM signal 300 arrives with atime offset 30, τ, and frequency offset 70, f, relative to the receiversampling. For proper reception, the frequency offset 70, f, must be muchsmaller than the tone spacing 1/T (74); and, as stated earlier inconnection with FIG. 2, the time offset 30, τ, must be smaller than thecyclic prefix length 44, T_(cp). The purpose of synchronization, asfurther described below, is to estimate these time and frequency offsetsto enable the receiver to align its sampling with the received signal.

[0029] Multitone Synchronization

[0030] With reference to FIGS. 1 through 4, FIG. 4 illustrates thetransmission and reception of the preferred multitone synchronizationsignals of the present invention. In order for the receiver 20 tosynchronize to the transmitter 10, the transmitter 10 sends a certainmultitone synchronization signal 500 as part of overall signal 350. Amultitone synchronization signal 500 is a signal transmitted in a timeinterval 84 preferably having the duration of a single OFDM symbolperiod 77 on some subset of the N tones 72. Using the number S to denotethe number of tones in the multitone synchronization signal 500, and foreach s-th tone, s=1, . . . , S, then n_(s) will denote a tone frequencyindex, and U_(s) will denote the complex value transmitted on the tone.FIG. 4 shows the time-frequency placement of an exemplary multitonesynchronization signal 500. In FIG. 4, the signal 500 has S=3 tones,whose frequency locations are indicated by the hatched areas 86.

[0031] It can be seen from the above that to conduct synchronization,the receiver should sample the overall signal 350 in a time intervalcontaining the multitone synchronization signal 500. Thissynchronization sample interval 62 must be sufficiently large as tofully contain the synchronization signal 500 for all possible timingoffsets 30, τ. Consequently, the receiver preferably has some a prioribound on a maximum time offset. This bound can be found from someprevious, coarser synchronization using any preferred, art recognizedtechnique, as a matter of design choice. In the case of a multipathchannel, for example, the sample interval 62 should be sufficientlylarge as to contain all possible received copies of the signal 350.

[0032] Two-Stage Synchronization

[0033] With continuing reference to FIGS. 1 through 4, and referringalso to FIG. 5, there is illustrated a proposed two-stage system 22 forsynchronizing the receiver 20 from the data captured in thesynchronization sample interval discussed above. The system 22 ispreferably configured to reside at or proximate the receiver 20. Thesystem could be implemented in a microprocessor, general purposecomputer, digital signal processor, other art-recognized platform, orsome combination of the aforementioned.

[0034] In the system of the present invention, a synchronizationinterval sampler 24 first extracts and samples the component of thesignal from the synchronization sample interval. As is known in the artof OFDM processing, the sampler 24 preferably uses a sample period ofT/N. The sequence of baseband, complex samples are denoted by y(m), m=0,. . . , M_(y)−1, where M_(y) denotes the total number of samples in thesynchronization sample interval. The synchronization system 22 also hasstored therein the values of a reference multitone synchronizationsignal in a read-only memory (ROM) 25. The reference synchronizationsignal can be stored in either the time or frequency domain format, aswill be further discussed herein.

[0035] After capturing the data from the synchronization interval 62, asdiscussed above, time and frequency offsets are estimated by locatingthe multitone signal 500 within the captured data. For preferredcomputational reasons, the time and frequency offset estimation isperformed in two stages. An initial time-frequency offset estimator 26yields a time offset estimate {circumflex over (τ)} and an initialfrequency offset estimate {circumflex over (f)}_(init). As will beexplained further hereinbelow, the initial estimation is performed by adiscrete search over a finite set of frequency candidates. Consequently,the frequency estimate may initially not be as accurate as may bedesired. To improve the frequency offset estimate, a frequency offsetrefinement block 28 performs a certain numerical optimization procedureyielding an improved frequency offset estimate denoted {circumflex over(f)}. The time and frequency offset estimates, {circumflex over (τ)} and{circumflex over (f)}, from the aforementioned two stages, are used bythe receiver 20 to synchronize to the received signal and perform theregular, art-recognized receiver tasks.

[0036] Initial Time and Frequency Offset Estimation

[0037] Turning now to FIG. 6, there is depicted a block diagram of asystem implementing the proposed initial time and frequency offsetestimation performed by estimator 26. Generally, the initial estimationis preferably obtained by conducting a discrete search over apre-selected set of candidate frequency offsets, {circumflex over (f)}₁,. . . , {circumflex over (f)}_(F). As discussed in greater detail below,the candidate frequency offsets can be taken from the range of possiblefrequency offsets. For each candidate frequency offset, {circumflex over(f)}_(i), an initial estimator searches the received signal samples,y(m), for a frequency shifted version of the multitone synchronizationsignal, u₀(m). This search yields estimates of the pilot signal energy,Ê_(i), and pilot signal time offset, {circumflex over (τ)}_(i),corresponding to the candidate frequency offset estimates, {circumflexover (f)}_(i). The initial estimator then selects the time and frequencyoffset estimate pair, ({circumflex over (τ)}_(i), {circumflex over(f)}_(i)), corresponding to the largest detected energy, Ê_(i).

[0038] The input y(m) is the sequence of time-domain samples from thesynchronization interval sampler 24 in FIG. 5. The input u₀ (m) is thesequence of time-domain samples of the reference multitonesynchronization signal, which can be loaded from ROM 25. Similar toy(m), the samples for u₀ (m) are preferably taken with the standard OFDMsampling period of T/N. If the multitone signal has S tones at frequencyindices n_(s) with complex values U_(s), the samples are given by${{u_{0}(m)} = {\sum\limits_{s = 1}^{S}{U_{s}^{2\pi \quad {{imn}_{s}/N}}}}},{m = {0\quad \ldots}}\quad,{M_{u} - 1},$

[0039] where M_(U)=┌NT_(s)/T┐ is the number of samples to cover oneT_(s)-length OFDM symbol period.

[0040] The reference multitone signal u₀(m) is multiplied by theexponentials, e^(2πi{circumflex over (f)}) _(^(i)) ^(Tm/N), to createfrequency shifted reference signals, u_(i)(m), i=1 , . . . , F.

[0041] The time-domain correlation (TDC) estimators 32 then search forthe frequency-shifted reference signals, u_(i)(m), within the receivedsignal, y(m). The search is performed via a smoothed time-domaincorrelation estimate that will be explained below. For each candidatefrequency offset estimate {circumflex over (f)}_(i), the smoothed TDC 32yields: {circumflex over (τ)}_(i), an estimate of the time offset of thefrequency shifted signal, u_(i)(m), within the received signal, y(m);and Ê_(i), an estimate of the reference signal energy, within thereceived signal.

[0042] After performing the smoothed TDC estimates, the selector block34 selects the estimate from the F candidate time-frequency offsetestimates, ({circumflex over (τ)}_(i), {circumflex over (f)}_(i)), i=1,. . . ,F corresponding to the maximum detected energy, Ê_(i).

[0043] Smoothed Time Domain Correlation Estimation for MultipathChannels

[0044] With reference to FIGS. 1 through 7, FIG. 7 illustrates anexemplary timing estimation problem for a multipath channel. In OFDMtransmission, it is known that certain channels may be multipath,meaning that signals from the transmitter can arrive at the receiver viaseveral different physical routes. In the synchronization system of thepresent invention, multipath channels result in the receiver 20receiving several copies of the multitone synchronization signal 500,each copy arriving at a different time. FIG. 7 illustrates an exemplarymultipath delay profile. The figure shows a number of multipath arrivaltimes 64 of the multitone synchronization signal 500 within thesynchronization sample interval 62, each arrival time being indicated bya vertical arrow. The height of the arrows indicate the relative energyof the copies arriving at each time.

[0045] Referring now to FIG. 8, in the system of the present invention,each of TDC estimators 32 comprises three functional blocks. The firstblock is a standard time-domain correlation (TDC) block 52 and computesR_(i)(m), the cross-correlation between u_(i)(m) and the receivedsignal. The cross-correlation magnitude, |R_(i)(m) |, is a standardestimate of the energy of the reference synchronization signal, ui(m),received at a time offset of m samples. The cross-correlation R_(i)(m)can be computed by standard FFT methods.

[0046] The second block, the smoothing filter 54, computes {overscore(R)}_(i)(m), the sum of the cross-correlation magnitudes, |{circumflexover (R)}_(i)(l)|, in a T_(cp)-length interval beginning at a timeoffset of m samples. This summation can be computed with a standardfinite impulse response (FIR) filter on the input |{overscore(R)}_(i)(l)|.

[0047] The final block, the maximum detector 56, computes, Ê_(i), themaximum value of the filtered output, {overscore (R)}_(i)(m), and{circumflex over (τ)}_(i), the time corresponding to the sample m atwhich {overscore (R)}_(i)(m) is maximized.

[0048] Now, since |R_(i)(m)| represents an estimate of the energy of thereference synchronization signal, u_(i)(m), received at a time offset ofm samples, the filtered cross-correlation, {overscore (R)}_(i)(m),represents an estimate of the total energy in a T_(cp)-length intervalat a time offset of m samples. Therefore, the maximum detector 56output, {circumflex over (τ)}_(i) is an estimate of the time τ at whichthe energy of the reference signal received in the time interval |τ,τ+T_(cp)| is maximized. The output Ê_(i) is an estimate of the totalreceived energy.

[0049] Simplified Initial Time and Frequency Offset Estimation

[0050]FIG. 9 is a block diagram of an alternative, simpler embodiment ofan initial time and frequency offset estimator 260. The initial time andfrequency estimator 26 in FIG. 6 requires more computing overhead thanestimator 260. Specifically estimator 26 requires F smoothed TDCestimators, and each TDC requires an M-point FFT followed by a smoothingoperation. Therefore, performing the procedure could be beyond thecomputational resources of certain receivers if F is large. The number,F, of candidate frequency offsets to test needs to be large when anaccurate estimate is required or the initial frequency range is large.

[0051] The system in FIG. 9 provides an alternative, computationallysimpler method for obtaining initial estimates, {circumflex over (τ)}and {circumflex over (f)}_(init), of the time and frequency offsets ofthe multitone signal u₀(m) in the received signal y(m). The basis ofthis simplified estimator is to first obtain a frequency offsetestimate, {circumflex over (f)}_(init), and then use the frequencyoffset estimate to obtain a time offset estimate, {circumflex over (τ)}.

[0052] For the frequency offset estimate, the system in FIG. 9 firstcomputes Y(n), by performing an FFT of the received signal y(m) in FFTblock 262. The system also uses an FFT of the multitone reference signalu₀(m). This output of FFT block 262 is denoted U₀(n), but this value canalso preferably be pre-computed and loaded from the synchronizationsignal ROM 25 in FIG. 5 (not shown).

[0053] With the FFTs computed, the frequency offset is easily estimatedby finding frequency offset at which the reference multitone signal andthe received signal are maximally correlated. To this end, afrequency-domain correlation estimator 264 sets the frequency estimateby the formula: {circumflex over (f)}_(init)−kΔf , where Δf is the FFTtone spacing, and k is the offset at which U₀(n+k) and Y(n) aremaximally correlated.

[0054] After determining a frequency offset estimate {circumflex over(f)}_(init) , the time offset, {circumflex over (τ)}, can be estimatedas before. That is, the reference signal can be shifted by the frequencyoffset estimate {circumflex over (f)}_(init), and then a smoothed TDC266 can be used to estimate the time offset of the frequency shiftedreference signal within the received signal. As before, the timeestimation can be performed with the FFTs U₀(n) and Y(n).

[0055] Frequency Offset Estimation Refinement

[0056]FIG. 10 shows a block diagram of a possible implementation of the“frequency offset estimate refinement” block in FIG. 5.

[0057] The first block 101, the T-length interval extractor has twoinputs: y(m), a sequence of received synchronization samples, and{circumflex over (τ)}, a time estimate. The output of this blockconsists of a subset of the y(m) sequence, which is defined as follows.The starting point of the subset sequence is τ away from the beginningof y(m) sequence. The length of the subset is equal to the interval ofuO(m), the reference multitone signal.

[0058] The second block 103, the numerical oscillator, generates asequence of complex samples e^(2π1m), (which is similar to what is shownin FIG. 6).

[0059] The output sequences of the first and the second blocks aremultiplied by multiplier 105 and the result is a sequence to be inputtedto the third block 107, the correlator. The other input of this block isthe reference sample sequence u0(m). The correlator block outputs thecorrelation of the two input sequences.

[0060] The last block 109, the numerical optimizer, takes thecorrelation input and adjusts the frequency estimate {circumflex over(f)}. Standard numerical optimization procedures can be used in thisblock. The adjusted frequency estimate is fed back to the second block,and the above procedure repeats.

[0061] More specifically, the frequency offset estimate, {circumflexover (f)}_(init), from the initial time and frequency offset estimationmay not in all circumstances be sufficiently accurate for the chosenapplication. As described above, the initial estimator is based ontesting discrete frequency offset candidates. In order that thecomputation in the initial estimation is not overwhelming, the number offrequency offset candidates, F, must be kept small. Also, in thesimplified estimator, the frequency can be estimated only within the FFTtone spacing. Consequently, a more accurate estimate of the frequencyoffset may be needed after the initial estimation has been performed.

[0062] The system in FIG. 10 can be used to find a more accuratefrequency offset estimate. The input y(m) to the system is thetime-domain samples of the received signal, and u₀(m) is the referencemultitone signal described previously. The system first uses the timeoffset estimate, {circumflex over (τ)}, computed in the T-lengthinterval extractor 101 initial estimation stage, to extract a T-lengthsample of y(m) containing the reference multitone signal.

[0063] A numerical oscillator 103 generates a complex exponential of acandidate frequency offset, {circumflex over (f)}. The T-length sampleof the received signal y(m) is then multiplied in multiplier 105 by theoscillator output, frequency shifting the received signal by {circumflexover (f)}. The frequency-shifted received signal is then correlated incorrelator 107 against the reference multitone signal u₀(m). Thiscorrelation can be performed by standard FFT methods. In particular, ifu₀(m) is a multitone signal with S tones, the correlation can becomputed from the corresponding S FFT outputs.

[0064] In principle, the true frequency offset is the frequency at whichthe correlation is maximized. A numerical optimization block 109recursively tests different frequency offsets {circumflex over (f)} andselects the frequency offset which maximizes the correlation. Theoptimization can be conducted with standard numerical optimizationprocedures using the frequency offset estimate {circumflex over(f)}_(init) from the initial time and frequency offset estimation as astarting point.

[0065] Application to OFDM Systems with Channel Estimation Pilot Signals

[0066]FIG. 11 shows an exemplary construction of a multitonesynchronization signal from an OFDM channel estimation pilot signal. Incertain OFDM systems, the transmitter sends a well-known pilot, orreference, signal from which any receiver can estimate the channel andcoherently demodulate the data. The OFDM channel estimation pilot signalis typically sent on some designated subset of the tones in designatedOFDM symbol periods. In a given OFDM symbol period, the tones used forthe channel estimation pilot signal are called “channel estimation pilottones”, or simply “pilot tones”. The remaining non-pilot tones are usedfor data transmission to the receivers. FIG. 11 shows an exampledistribution of channel estimation pilot tones in the time-frequencygrid. The pilot tones are indicated by the hatched regions 420.

[0067] In OFDM systems with channel estimation pilot signals, thechannel estimation pilot signal can also be used for timing andfrequency synchronization. To this end, a receiver first coarselysynchronizes to the channel estimation pilot signal, approximatelylocating it in time and frequency. The receiver then selects one of theOFDM symbols and uses the channel estimation pilot tones within thesymbol as a multitone synchronization signal. In the example depicted inFIG. 11, the pilot tones selected for use as synchronization tones areindicated by the solid intervals 450. Any OFDM symbol containing channelestimation pilot tones can be used. The receiver can then follow theteachings herein described above and estimate the time and frequencyoffsets accurately from the multitone synchronization signal. In thisway, the receiver can obtain synchronization without having thetransmitter send any pilots in addition to the pilot used for channelestimation.

[0068] Thus the instant invention offers benefits over prior artsystems. For example, the multitone signals are a well suited choice forOFDM synchronization pilots, since they can be transmitted on tonesdistinct from the data tones so that they do not interfere with theregular data transmission. Also, many existing or proposed OFDM systemsperiodically transmit multitone signals as channel estimation pilots inmanners known in the art. These multitone channel estimation pilotscould also be used for the purpose of timing and frequencysynchronization. In the prior art, it was necessary to transmit thesynchronization pilot signals in addition to the channel estimationpilots.

[0069] Thus, as has been set forth above, one feature of the presentinvention is that the time and frequency synchronization can beperformed jointly in a computationally efficient manner. Specifically,the computational load is reduced by using a two-stage procedure ofcoarse estimation followed by frequency estimate refinement. Also, inthe proposed simplified implementation search method, the frequencyoffset is estimated before the time offset and the joint two-dimensionalsearch is avoided.

[0070] Additionally, the timing synchronization method is well-suited tomultipath channels. The smoothed TDC estimator presented herein locatesa cyclic prefix length interval which captures the maximum receivedsignal energy. The location of the interval is estimated withoutestimating the individual path's locations.

[0071] The system and methods taught herein can be utilized in a widevariety of communications systems, whether over wired, wireless,ultrasonic, optical, laser or other art recognized channels or media,including underwater. The system may be implemented as discretecomponents, integrated components, application specific integratedcircuits, in software, hardware or firmware, in a digital signalprocessor, microprocessor, or as a combination of one or more of theaforementioned implementation methodologies, or otherwise, as a matterof design choice.

[0072] Thus, while there have been shown and described and pointed outfundamental novel features of the invention as applied to preferredembodiments thereof, it will be understood that various omissions andsubstitutions and changes in the form and details of the disclosedinvention may be made by those skilled in the art without departing fromthe spirit of the invention. It is the intention, therefore, to belimited only as indicated by the scope of the claims appended hereto.

What is claimed is:
 1. An orthogonal frequency division multiplexing(OFDM) communication device, comprising: an OFDM receiver for receivingan OFDM signal containing a multitone synchronization signal; asynchronization interval sampler coupled to said receiver; an initialtime and frequency offset estimator connected to said sampler and saidreceiver; and a frequency offset estimate refinement unit connected tosaid receiver, said sampler and said estimator, wherein a referencemultitone synchronization signal is used by said estimator and saidrefinement device in calculating a time offset and a frequency offset ofsaid multitone synchronization signal, said receiver utilizing said timeoffset and said frequency offset to synchronize with said received OFDMsignal.
 2. The system of claim 1, wherein said initial time andfrequency offset estimator comprises: a plurality of smoothedtime-domain correlation estimators for outputting a series of timeoffset estimate and correlation estimate pairs, each pair related to afrequency offset estimate; and a selector for selecting a selected timeoffset estimate and a selected initial frequency offset based in partupon the selection of the frequency offset estimate and time offsetestimate that corresponds with the largest value of correlationestimate. The system of claim 2, wherein each of said smoothed timedomain correlation estimators comprises: a time domain correlator; asmoothing filter connected to said time domain correlator and receivingan output from said time domain correlator; and a maximum detectorconnected to and receiving an output from said smoothing filter fordetecting a signal energy maxima representing a time estimate at whichthe energy of said reference multitone synchronization signal is at amaximum.
 3. The system of claim 2, wherein the initial time andfrequency offset estimator uses a coarse frequency discretization usingF candidate frequency offsets.
 4. The system of claim 2, wherein saidreference multitone synchronization signal has a length of T, andwherein said frequency offset estimate refinement device comprises: aT-length interval extractor for extracting a T-length sample of theoutput of said sampler; a numerical oscillator for generating a complexexponential of a candidate frequency offset; a multiplier formultiplying said T-length sample with said complex exponential to obtaina frequency shifted received signal; a correlator for correlating saidfrequency shifted received signal with said reference multitonesynchronization signal and producing a correlation output; and anumerical optimizer for receiving said correlation output and outputtinga new frequency offset candidate.
 5. The system of claim 5, wherein saidnew frequency offset candidate and a time offset associated with saidnew frequency offset candidate are used by said receiver if said newfrequency offset candidate is a candidate that yields a maximumcorrelation output.
 6. The system of claim 1, wherein said initial timeand frequency offset estimator comprises: a first Fast FourierTransformer for obtaining a transform of said received signal; an secondFast Fourier Transformer device for obtaining a transform of saidreference multitone synchronization signal; a frequency domaincorrelation estimator for receiving said received signal transform andsaid reference signal transform and outputting an initial frequencyoffset estimate; and
 7. A time domain correlation estimator forreceiving said received signal transform and said reference signaltransform and said initial frequency offset estimate and outputting atime offset estimate.
 8. A method of synchronizing an orthogonalfrequency division multiplexing (OFDM) receiver with a received OFDMsignal comprising a multitone synchronization signal, comprising thesteps of: obtaining a coarse time offset estimate of said receivedsignal; sampling said received signal in a selected time interval toderive samples of said multitone synchronization signal; analyzing saidsamples with respect to a reference multitone synchronization signal toobtain, for each sample analyzed, a time offset, a frequency offset, anda signal energy; selecting a one of said analyzed samples with thegreatest signal energy to yield a selected time offset estimate and aselected frequency offset estimate for use by said receiver insynchronizing with said received OFDM signal.
 9. The method of claim 8,further comprising passing said selected time offset estimate and saidselected frequency offset estimate to said receiver for use by saidreceiver in sychronizing with said received OFDM signal.
 10. A method ofcarrying out OFDM communications comprising: receiving an OFDM signalincluding within it a multitone synchronization signal; locating saidsynchronization signal within said OFDM signal; determining a timeoffset value of said synchronization signal; determining an initialfrequency offset value of said synchronization signal; and recursivelyrefining said frequency offset estimate to yield a selected pair of timeand frequency offset values to be used by said OFDM receiver.
 11. Themethod of claim 10, wherein said initial time offset value and saidinitial frequency offset value are determined by obtaining a correlationwith a stored reference value of said synchronization signal.
 12. Themethod of claim 11, wherein said correlation is performed seeking amaximum received synchronization signal energy level.
 13. A method ofcarrying out OFDM communications comprising: receiving, in an OFDMreceiver, an OFDM signal including within it a multitone synchronizationsignal; obtaining an FFT transform of said received signal; obtaining anFFT transform of said reference multitone synchronization signal;correlating said received signal transform and said reference signaltransform and outputting an initial frequency offset estimate when saidaforementioned transforms are maximally correlated; and correlating saidreceived signal transform and said reference signal transform and saidinitial frequency offset estimate and outputting a time offset estimatewhen said aforementioned transforms are maximally correlated.
 14. Anorthogonal frequency division multiplexing (OFDM) communication device,comprising: means for receiving an OFDM signal containing a multitonesynchronization signal; means, coupled to said receiving means, forsampling a synchronization interval of said OFDM signal; means,connected to said sampling means and said receiving means, for obtainingan initial time estimate and an initial frequency offset estimate ofsaid OFDM signal; means, connected to said receiving means, saidsampling means and said estimating means, for obtaining a frequencyoffset estimate refinement; and storage means, connected to saidestimating means and said refinement means, for storing a referencemultitone synchronization signal for use by said estimating means andsaid refinement means in calculating a time offset and a frequencyoffset of said multitone synchronization signal, said receiving meansutilizing said time offset and said frequency offset to synchronize withsaid received OFDM signal.
 15. The system of claim 14, wherein saidestimating means further comprises: a plurality of means for obtainingsmoothed time-domain (TDC) correlation estimates, said smoothed TDCestimate means outputting a series of time offset estimate andcorrelation estimate pairs, each pair related to a frequency offsetestimate; and means for selecting a selected time offset estimate and aselected initial frequency offset based in part upon the selection ofthe frequency offset estimate and time offset estimate that correspondswith the largest value of correlation estimate.
 16. The system of claim15, wherein each of said smoothed TDC estimate means comprises: a timedomain correlator; a smoothing filter connected to said time domaincorrelator and receiving an output from said time domain correlator; anda maximum detector connected to and receiving an output from saidsmoothing filter for detecting a signal energy maxima representing atime estimate at which the energy of said reference multitonesynchronization signal is at a maximum.
 17. The system of claim 16,wherein the estimating means uses a coarse frequency discretizationusing F candidate frequency offsets.
 18. The system of claim 15, whereinsaid reference multitone synchronization signal has a length of T, andwherein said refinement means comprises: a T-length interval extractorfor extracting a T-length sample of the output of said sampler; anumerical oscillator for generating a complex exponential of a candidatefrequency offset; a multiplier for multiplying said T-length sample withsaid complex exponential to obtain a frequency shifted received signal;a correlator for correlating said frequency shifted received signal withsaid reference multitone synchronization signal and producing acorrelation output; and a numerical optimizer for receiving saidcorrelation output and outputting a new frequency offset candidate. 19.The system of claim 18, wherein said new frequency offset candidate anda time offset associated with said new frequency offset candidate areused by said receiving means if said new frequency offset candidate is acandidate that yields a maximum correlation output.
 20. The system ofclaim 14, wherein said estimating means comprises: first means forobtaining a first Fast Fourier Transform (FFT) of said received signal;second means for obtaining a Fast Fourier Transform of said referencemultitone synchronization signal; frequency domain correlation estimatemeans for receiving said received signal transform and said referencesignal transform and outputting an initial frequency offset estimate;and time domain correlation estimator means for receiving said receivedsignal transform and said reference signal transform and said initialfrequency offset estimate and outputting a time offset estimate.
 21. Adevice for synchronizing an orthogonal frequency division multiplexing(OFDM) receiver with a received OFDM signal comprising a multitonesynchronization signal, comprising: means for obtaining a coarse timeoffset estimate of said received signal; means for sampling saidreceived signal in a selected time interval to derive samples of saidmultitone synchronization signal; means for analyzing said samples withrespect to a reference multitone synchronization signal to obtain, foreach sample analyzed, a time offset, a frequency offset, and a signalenergy; and means for selecting one of said analyzed samples with thegreatest signal energy to yield a selected time offset estimate and aselected frequency offset estimate, wherein said selected time offsetestimate and said selected frequency offset estimate are used by saidreceiver in synchronizing with said received OFDM signal.
 22. The deviceof claim 21, further comprising means for passing said selected timeoffset estimate and said selected frequency offset estimate to saidreceiver for use by said receiver in synchronizing with said receivedOFDM signal.
 23. A device for carrying out OFDM communicationscomprising: means for receiving an OFDM signal including within it amultitone synchronization signal; means for locating saidsynchronization signal within said OFDM signal; means for determining atime offset value of said synchronization signal; means for determiningan initial frequency offset value of said synchronization signal; andmeans for recursively refining said frequency offset estimate to yield aselected pair of time and frequency offset values to be used by saidOFDM receiver.
 24. The device of claim 23, wherein said initial timeoffset value and said initial frequency offset value are determined byobtaining a correlation with a stored reference value of saidsynchronization signal.
 25. The device of claim 24, wherein saidcorrelation is performed seeking a maximum received synchronizationsignal energy level.
 26. A system for carrying out OFDM communicationscomprising: means for receiving an OFDM signal including within it amultitone synchronization signal means for obtaining an FFT transform ofsaid received signal; means for obtaining an FFT transform of saidreference multitone synchronization signal; means for correlating saidreceived signal transform and said reference signal transform andoutputting an initial frequency offset estimate when said aforementionedtransforms are maximally correlated; and means for correlating saidreceived signal transform and said reference signal transform and saidinitial frequency offset estimate and supplying as an output a timeoffset estimate when said aforementioned transforms are maximallycorrelated.
 27. An OFDM signal processor comprising: an OFDM receiverfor receiving an OFDM signal containing a multitone synchronizationsignal; a synchronization interval sampler connected to said input andsaid receiver; an initial time and frequency offset estimator connectedto said sampler and said receiver; and a frequency offset estimaterefinement device connected to said receiver, said sampler and saidestimator, wherein a reference multitone synchronization signal is usedby said estimator and said refinement device in calculating a timeoffset and a frequency offset of said multitone synchronization signal,said receiver utilizing said time offset and said frequency offset tosynchronize with said received OFDM signal.
 28. The processor of claim27, wherein said reference multitone synchronization signal is storedfor retrieval in a memory connected to said estimator and saidrefinement device.
 29. The processor of system of claim 27, wherein saidinitial time and frequency offset estimator comprises: a plurality ofsmoothed time-domain correlation estimators for outputting a series oftime offset estimate and correlation estimate pairs, each pair relatedto a frequency offset estimate; and a selector for selecting a selectedtime offset estimate and a selected initial frequency offset based inpart upon the selection of the frequency offset estimate and time offsetestimate that corresponds with the largest value of correlationestimate.
 30. The processor of claim 29, wherein each of said smoothedtime domain correlation estimators comprises: a time domain correlator;a smoothing filter connected to said time domain correlator andreceiving an output from said time domain correlator; and a maximumdetector connected to and receiving an output from said smoothing filterfor detecting a signal energy maxima representing a time estimate atwhich the energy of said reference multitone synchronization signal isat a maximum.
 31. The processor of claim 29, wherein the initial timeand frequency offset estimator uses a coarse frequency discretizationusing F candidate frequency offsets.
 32. The processor of claim 29,wherein said reference multitone synchronization signal has a length ofT, and wherein said frequency offset estimate refinement devicecomprises: a T-length interval extractor for extracting a T-lengthsample of the output of said sampler; a numerical oscillator forgenerating a complex exponential of a candidate frequency offset; amultiplier for multiplying said T-length sample with said complexexponential to obtain a frequency shifted received signal; a correlatorfor correlating said frequency shifted received signal with saidreference multitone synchronization signal and producing a correlationoutput; and a numerical optimizer for receiving said correlation outputand outputting a new frequency offset candidate.
 33. The processor ofclaim 32, wherein said new frequency offset candidate and a time offsetassociated with said new frequency offset candidate are used by saidreceiver if said new frequency offset candidate is a candidate thatyields a maximum correlation output.
 34. The processor of claim 32,wherein said initial time and frequency offset estimator comprises: afirst Fast Fourier Transformer for obtaining a transform of saidreceived signal; an second Fast Fourier Transformer device for obtaininga transform of said reference multitone synchronization signal; afrequency domain correlation estimator for receiving said receivedsignal transform and said reference signal transform and outputting aninitial frequency offset estimate; and a time domain correlationestimator for receiving said received signal transform and saidreference signal transform and said initial frequency offset estimateand outputting a time offset estimate.
 35. An OFDM transmittercomprising: means for transmitting an OFDM signal comprising a firsttime interval and a second time interval; means for transmitting data atone or more data frequencies during said first time interval; and meansfor transmitting, during said second time interval, a synchronizationtone, at one or more synchronization frequencies, for a predeterminedtime period, the frequencies of said synchronization tone being distinctfrom said data frequencies.
 36. A method for transmitting an OFDM signalcomprising the steps of: transmitting an OFDM signal comprising a firsttime interval and a second time interval; means for transmitting data atone or more data frequencies during said first time interval; and meansfor transmitting, during said second time interval, a synchronizationtone, at one or more synchronization frequencies, for a predeterminedtime period, the frequencies of said synchronization tone being distinctfrom said data frequencies.